Fuel injection arrangements of the kind referred to above utilize the fluctuating crankcase pressure for driving an injection pump and for injecting fuel. The pressure in the crankcase is dependent upon the rotational speed and the load of the two-stroke engine. An overpressure develops in the crankcase during the downward movement of the piston in the direction toward bottom dead center; whereas, with the following upward movement of the piston to top dead center, the crankcase pressure drops to an underpressure. The crankcase pressure fluctuates thereby between positive and negative values with the positive values likewise increasing to a maximum with increasing speed and the positive values then remain constant up to the highest speed. The pressure fluctuations lie, for example, approximately between 0.75 and -0.2 bar.
German published patent application No. 2,248,584 discloses a fuel injection arrangement wherein the increasing pressure in the crankcase operates on a pressure receiving surface of the pump piston and displaces the latter to carry out a suction stroke. Fuel is drawn into the pump chamber which is secured by check valves. The positive crankcase pressure drops in the region of bottom dead center so that the pulse chamber becomes pressureless and the pump piston carries out a pumping stroke actuated by the force of the spring. This pumping stroke lies about or shortly after bottom dead center and effects the injection of fuel into the combustion chamber. A late injection of fuel of this kind may be adequate for slow-running two-stroke engines. However, this type of injection is unsuitable for fast-running two-stroke engines.
For the above reason, the initiation of the injection operation is largely decoupled from the course of the pressure in fuel injection arrangements of the kind discussed above. For decoupling, a relief opening is provided which discharges the pulse chamber into the combustion chamber as soon as the relief opening is reached by the movable part of the two-stroke engine. The increasing crankcase pressure is conducted to the pulse chamber via the control opening in the range of the crankshaft angle of 80.degree. to 100.degree. and the control opening is closed before bottom dead center is reached. Significantly in advance of bottom dead center, a discharge of the pulse chamber into the combustion chamber is possible via the relief opening whereby the injection is initiated. This injection then lies ahead of bottom dead center and is also applicable to fast-running two-stroke engines. Experience has shown that it is especially in fast-running two-stroke engines that short-term conditions can occur which effect a high pressure in the combustion chamber at the point in time that the relief opening is controlled. Such conditions can occur with delayed combustion, incomplete combustion or the like. The high pressure acts on the pulse chamber via the relief opening and causes the pump piston to be displaced still further whereby its suction stroke and thereby the quantity of fuel drawn in by suction is increased. The two-stroke engine receives a quantity of fuel which is too great in the injection which follows which, in turn, leads to irregular conditions in the combustion chamber and thereby to a build-up of a combustion error.